A switched capacitor filter (SFC) circuit is a filter circuit having a low cutoff frequency. The SFC circuit has a simple structure and consists of an analog switch, a capacitor, and an operational amplifier (op-amp). Therefore, the SFC circuit is suitable for use in a signal processing circuit integrated on one chip as a monolithic integrated circuit (IC).
For example, a capacitive physical quantity sensor disclosed in U.S. Pat. No. 6,483,322 by the present applicant includes a SFC circuit incorporated in a signal processing circuit that processes an output voltage of a capacitance-to-voltage converter and outputs a signal corresponding to a change in physical quantity. The SFC circuit eliminates harmonic noise from the output voltage of the capacitance-to-voltage converter.
Since a SFC circuit achieves predetermined filter characteristics by switching an analog switch, the switched capacitor filter circuit may produce harmonic noise, i.e., switching noise. Therefore, a low pass filter circuit is generally added to an output of the SFC circuit to eliminate the switching noise from an output signal of the SFC circuit.
Typically, the low pass filter circuit is implemented by an analog active filter circuit, because the analog active filter circuit does not produce the switching noise. Further, the use of the analog active filter circuit allows the monolithic IC chip to be compact.
An analog active filter includes a capacitor and a resistor. The rise time of an output signal of the analog active filter circuit is determined by a time constant depending on capacitance of the capacitor and resistance of the resistor. There is a delay time between when a power supply of the analog active filter circuit is turned on and when the output signal of the analog active filter circuit reaches its final value. Therefore, the analog active filter circuit cannot be used in applications that require fast start-up operation or fast intermittent operation where the power supply is repeatedly turned on and off at very short intervals.